Process for producing thermoplastic resin sheet with controlled warpage

ABSTRACT

The present invention provides a process for producing a thermoplastic resin sheet with controlled warpage, the process including extruding a thermoplastic resin sheet, while giving mechanically controlled warpage to the sheet at a position where a temperature of the sheet extruded becomes around a grass transition temperature (Tg) of a thermoplastic resin constituting the sheet. In this production process, the sheet is preferably allowed to pass through a pair of warpage-controlling upper and lower rolls at the position, and the mechanically controlled warpage is preferably given to the sheet by selecting shapes of the warpage-controlling rolls and/or by disposing the warpage-controlling rolls on an upper side or a lower side of a height where a flat sheet is obtained. According to this production process, a thermoplastic resin sheet with controlled warpage can be produced simply and easily with high efficiency.

BACKGROUND OF THE INVENTION

The present application claims the benefit of priority from Japanese Patent Application No. 2006-115603, filed on Apr. 19, 2006, all the contents of which are incorporated herein by reference.

1. Field of the Invention

The present invention relates to a process for producing a thermoplastic resin sheet with controlled warpage.

2. Description of the Related Art

Thermoplastic resin sheets are produced by, for example, an extrusion method. In this case, a thermoplastic resin sheet is obtained in such a manner that a resin composition is melted by heating and extruded through a die, after which the extruded sheet is extended through cooling rolls by applying pressure and allowed to pass through guide rolls, and then solidified by cooling, while being taken up on a take-up roll or the like. When a thermoplastic resin sheet is produced by an extrusion method, various inventions have been made in general for the purpose of suppressing warpage to obtain a flat sheet.

For example, Japanese Patent Laid-open (Kokai) Publication No. Hei 6-344417 discloses a process for relaxing internal stress through slowly cooling after raising temperature by adjusting the temperature of cooling rolls and heating an extruded sheet. Japanese Patent Laid-open (Kokai) Publication No. Hei 7-276471 discloses a process for relaxing internal stress by giving flexure to a sheet between the final cooling roll and the first guide roll. Japanese Patent Laid-open (Kokai) Publications Nos. 2001-139705 and 2005-81757 disclose a process for suppressing the generation of internal stress by adjusting the speed ratio of the second roll and the third roll, and setting the roll temperatures to be relatively low. Japanese Patent Laid-open (Kokai) Publication No. 2004-126185 discloses an improved process in which the processes disclosed in Japanese Patent Laid-open (Kokai) Publications Nos. Hei 7-276471 and 2001-139705 are used in combination.

In this way, the prior arts have mainly aimed at obtaining a flat sheet through suppressing warpage by suppressing the generation of internal stress or relaxing internal stress.

However, in some applications of thermoplastic resin sheets, there are cases that warpage is rather required to be given intentionally. For example, light diffusion sheets used in backlight units of liquid crystal display devices or monitors for medical use, even if they are flat at the time of their production, may absorb moisture in the air and then be dried and shrunk by the heat of a light on the side facing the light, so that convex warpage is generated towards the display surface side, which causes a problem that display performance may adversely be influenced when the backlight units are used.

Further, in applications of building materials, when applied to an R-shaped part, a building material with intended warpage is better in fitting to a substrate as compared with a flat building material without warpage, and a stress loaded on the building material is also reduced. When a thermoplastic resin sheet is produced as a building material with warpage, there has been adopted a process in which a flat sheet is extruded and warpage is then given to the sheet by hot forming.

However, this process needs another step of hot forming in addition to the extrusion step, which causes a problem that production costs may be increased for building materials.

Therefore, Japanese Patent Laid-open (Kokai) Publication No. 2002-120249 proposes a process in which using a production line for thermoplastic resin sheets, when the temperature of a thermoplastic resin sheet transferred from cooling rolls is within a specific range, the sheet is heated or cooled to control a difference in temperatures between the upper and lower surfaces of the sheet, thereby adjusting the amount of warpage of the sheet.

However, in this process, it may be difficult to precisely control a difference in temperatures between the upper and lower surfaces of the sheet, and it may be difficult to produce a thermoplastic resin sheet with controlled warpage.

SUMMARY OF THE INVENTION

Under these circumstances, it is an object of the present invention to provide a process for producing a thermoplastic resin sheet with controlled warpage simply and easily with high efficiency.

The present inventors have made various studies, and as a result, they have found that when a thermoplastic resin sheet is extruded, mechanically controlled (and also in some cases thermally controlled) warpage can be given to the sheet at a position where a temperature of the extruded sheet becomes around a glass transition temperature (Tg) of a thermoplastic resin constituting the sheet, thereby completing the present invention.

Thus, the present invention provides a process for producing a thermoplastic resin sheet with controlled warpage (hereinafter referred to in some cases as “production process of the present invention”), the process comprising extruding a thermoplastic resin sheet, while giving mechanically controlled warpage to the sheet at a position where a temperature of the extruded sheet becomes around a glass transition temperature (Tg) of a thermoplastic resin constituting the sheet.

In the production process of the present invention, the sheet may preferably be allowed to pass through a pair of warpage-controlling upper and lower rolls at the position, and the mechanically controlled warpage is given to the sheet by selecting shapes of the warpage-controlling rolls and/or by disposing the warpage-controlling rolls on an upper side or a lower side of a height where a flat sheet is obtained. In addition, the temperature of the sheet at a position of the warpage-controlling rolls may preferably be within a range of from (Tg−20° C.) to (Tg+20° C.). Further, thermally controlled warpage may also be given to the sheet by adjusting respective temperatures of the warpage-controlling rolls to generate a difference in cooling rates between the upper and lower surfaces of the sheet and/or by adjusting a difference in temperatures between the upper and lower surfaces of the sheet at a position or in a vicinity of the warpage-controlling rolls to generate a difference in cooling rates between the upper and lower surfaces of the sheet.

In the production process of the present invention, the thermoplastic resin constituting the sheet may preferably be selected from the group consisting of polycarbonate resins, (meth)acrylic resins, styrene resins, acryl-styrene resins, and norbornene resins, and may contain fine particles to have light diffusion property.

The present invention further provides a thermoplastic resin sheet with controlled warpage, which is obtainable by the above-described production process.

According to the present invention, a thermoplastic resin sheet with controlled warpage can be produced simply and easily with high efficiency. When the thermoplastic resin sheet obtained is used in, for example, a backlight unit of a liquid crystal display device as a light diffusion sheet, if slight warpage is given towards the light side beforehand so that it is design to become flat by the heat of the light at the time of use, there is no possibility that a liquid crystal display panel is pushed by warpage generating towards the display side, which gives no adverse influence on the display performance of the liquid crystal display device . Further, similarly, even in a backlight unit of a monitor for medical use, if slight warpage is given towards the light side beforehand so that it is design to become flat by the heat of the light at the time of use, no adverse influence is given on the display performance of the monitor for medical use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the structure of a typical sheet extruder which can be used in the production process of the present invention.

FIG. 2 is a schematic view showing the case where a pair of warpage-controlling upper and lower rolls is shifted to an upper side so that convex warpage is upwardly given to a thermoplastic resin sheet which is produced using the sheet extruder shown in FIG. 1.

FIG. 3 is a schematic view showing the case where a pair of warpage-controlling upper and lower rolls is shifted to a lower side so that convex warpage is downwardly given to a thermoplastic resin sheet which is produced using the sheet extruder shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

<<Process for Producing Thermoplastic Resin Sheet with Controlled Warpage>>

The production process of the present invention comprises extruding a thermoplastic resin sheet, while giving mechanically controlled warpage to the sheet at a position where a temperature of the extruded sheet becomes around a glass transition temperature (Tg) of a thermoplastic resin constituting the sheet. The term “warpage” as used herein means displacement from the case where a thermoplastic resin sheet is flat, and specifically, it means a distance from the center part of a thermoplastic resin sheet to threads when the sheet is in a condition to be hung down and the threads are strung between the centers of opposite sides of the sheet in the machine direction of extrusion (hereinafter referred to in some cases as “longitudinal direction”) and in the transverse direction of extrusion (hereinafter referred to in some cases as “lateral direction”). When the sheet warps towards the upper surface side (i.e., when warpage is upwardly convex), the warpage is denoted as “+”, and when the sheet warps towards the lower surface side (i.e., when warpage is downwardly convex), the warpage is denoted as “−”.

In the production process of the present invention, the amount of warpage may appropriately be set according to applications, although it is not particularly limited, and specifically, it is within a range of not smaller than about −100 mm and not greater than about −5 mm or not smaller than about +5 mm and not greater than about +100 mm. The warpage of a thermoplastic resin sheet is given along the longitudinal direction and/or along the lateral direction. The warpage given along the longitudinal direction means that, when the sheet is observed from the lateral direction, the sheet looks convex upwardly or downwardly, and the warpage given along the lateral direction means that, when the sheet is observed from the longitudinal direction, the sheet looks convex upwardly or downwardly, and the warpage given in the longitudinal and lateral directions means that the sheet convexly warps upwardly or downwardly towards the center part from the periphery parts of the sheet. In the case where a thermoplastic resin sheet has warpage in the longitudinal and lateral directions, depending on applications, the amount of warpage may preferably be on the same level in the longitudinal and lateral directions.

In the production process of the present invention, the above-described sheet may preferably be allowed to pass through a pair of warpage-controlling upper and lower rolls, and the mechanically controlled warpage may preferably be given to the sheet by selecting the shapes of the warpage-controlling rolls and/or by disposing the warpage-controlling rolls on an upper side or a lower side of a height where a flat sheet is obtained. The temperature of the sheet at a position of the warpage-controlling rolls may preferably be within a range of from (Tg−20° C.) to (Tg+20° C.). When the temperature of the sheet is lower than (Tg−20° C.), it may be difficult to give warpage to the sheet because the temperature of the sheet is too low. In contrast, when the temperature of the sheet is higher than (Tg+20° C.), the amount of warpage given to the sheet may change because the temperature of the sheet is too high, so that desired warpage cannot be given to the sheet.

In addition, thermally controlled warpage may also be given to the sheet by adjusting the respective temperatures of the warpage-controlling rolls to generate a difference in cooling rates between the upper and lower surfaces of the sheet and/or by adjusting a difference in temperatures between the upper and lower surfaces of the sheet at a position or in a vicinity of the warpage-controlling rolls to generate a difference in cooling rates between the upper and lower surfaces of the sheet.

In the production process of the present invention, warpage along the longitudinal direction of a sheet can be given by disposing a pair of warpage-controlling upper and lower rolls used in extrusion on an upper side or a lower side of a height where a flat sheet is obtained. Further, as a pair of warpage-controlling upper and lower rolls, warpage along the lateral direction of a sheet can be given by using, for example, a roll having a shape such as crowning, reverse crowning, or arch. Moreover, warpage can also be given by adjusting the respective temperatures of a pair of warpage-controlling upper and lower rolls to generate a difference in cooling rates on the upper and lower surfaces of the sheet. In addition to the control of warpage with such warpage-controlling rolls, a difference in temperatures between the upper and lower surfaces of a sheet may be adjusted by heating and/or cooling the sheet at a position downstream from the warpage-controlling rolls in the machine direction of extrusion to generate a difference in cooling rates on the upper and lower surfaces of the sheet. A position where a difference in temperatures between the upper and lower surfaces of a sheet is adjusted by heating and/or cooling the sheet may preferably be as near the warpage-controlling rolls as possible.

As described above, larger warpage can be given when the adjustment of a difference in temperatures between the upper and lower surfaces of the sheet is used in addition to the control of warpage with warpage-controlling rolls. Larger warpage can be given by, for example, using a roll having a reverse crowning shape as a warpage-controlling upper roll and a roll having a crowning shape as a warpage-controlling lower roll, and/or disposing the warpage-controlling rolls on an upper side of a height where a flat sheet is obtained, and setting the temperature of the warpage-controlling upper roll to be lower than that of the warpage-controlling lower roll, cooling the upper surface of a sheet having passed through the warpage-controlling rolls, and/or heating the lower surface of the sheet; or alternatively, by using a roll having a crowning shape as a warpage-controlling upper roll and a roll having a reverse crowning shape as a warpage-controlling lower roll, and/or disposing the warpage-controlling rolls on a lower side of a height where a flat sheet is obtained, and setting the temperature of the warpage-controlling upper roll to be higher than that of the warpage-controlling lower roll, heating the upper surface of a sheet having passed through the warpage-controlling rolls, and/or cooling the lower surface of the sheet.

The production process of the present invention is a process in which a thermoplastic resin sheet is extruded, while controlled warpage is given to the extruded sheet depending on the height, shapes, and temperatures of warpage-controlling rolls, and therefore, it is easy and certain to control warpage, and a thermoplastic resin sheet with controlled warpage can be produced simply and easily with high efficiency, as compared with a process disclosed in Japanese Patent Laid-open (Kokai) Publication No. 2002-120249, that is, a process in which the amount of warpage on a sheet is adjusted through controlling a difference in temperatures between the upper and lower surfaces of the sheet by heating or cooling the sheet.

As the material of a thermoplastic resin sheet, there can be mentioned, for example, polycarbonate resins; (meth)acrylic resins such as poly(methyl methacrylate); styrene resins such as polystyrene; acryl-styrene resins; and norbornene resins. In these thermoplastic resins, polycarbonate resins may particularly be preferred.

A thermoplastic resin sheet may be formed from a single material or from two or more kinds of materials, and may be composed of a single layer or two or more layers.

To a thermoplastic resin constituting a sheet, an additive(s) may be added, such as stabilizers, antioxidants, antistatic agents, plasticizers, dispersants, fluorescent brighteners, and diffusing agents. The amounts of these additives to be added may appropriately be adjusted depending on their kinds and the like, although they are not particularly limited.

Alternatively, on at least one side of a thermoplastic resin sheet, there may be provided a thin layer containing an additive(s) such as antioxidants, antistatic agents, ultraviolet absorbers, fluorescent brighteners, and diffusing agents. These additives may be contained all in a single thin layer or contained in two or more layers separately or in combination.

As the material constituting a thin layer, there can be mentioned, for example, (meth)acrylic resins, polyester resins, epoxy resins, and silicone resins. These resins may be used alone, or two or more kinds of these resins may also be used in combination. In these resins, (meth)acrylic resins may be preferred. In this case, instead of adding an antistatic agent(s) and an ultraviolet absorber(s), or in addition to adding an antistatic agent(s) and an ultraviolet absorber(s), (meth)acrylic resins having antistatic property and/or (meth)acrylic resins having ultraviolet absorption property may be used.

The thickness of a thin layer (in the case of two or more thin layers, the thickness of each thin layer) may preferably be 100 μm or smaller, more preferably 50 μm or smaller. When the thickness of a thin layer is greater than 100 μm, unintended warpage may occur due to differences in thermal shrinkage rate and water absorption rate in the case where a material different from a thermoplastic resin sheet is used. The thickness of a thin layer is an average value of ten data measured in such a manner that a thermoplastic resin sheet having a thin layer formed thereon is sliced at arbitrary ten points with a microtome to a 15 μm thick each, of which sections are observed with a microscope to measure the thickness of a thin layer.

To a thin layer, an additive(s) may be added, such as stabilizers, antioxidants, plasticizers, and dispersers. The amounts of these additives to be added may appropriately be adjusted depending on their kinds and the like, although they are not particularly limited.

The thickness of a thermoplastic resin layer (in the case where a thin layer is formed thereon, the thickness including the thin layer) may preferably be not smaller than 0.5 mm and not greater than 15 mm, more preferably not smaller than 1 mm and not greater than 10 mm. When the thickness is smaller than 0.5 mm, the mechanical strength of a thermoplastic resin sheet may be lowered. In contrast, when the thickness is greater than 15 mm, the extrusion of a thermoplastic resin sheet may become difficult, thereby lowering the quality of the sheet.

When a thermoplastic resin sheet is used as a light diffusion sheet, for example, transparent particles, and if necessary, a fluorescent brightener(s) and an antioxidant(s), are added at each suitable amount to a thermoplastic resin as described above. The haze of the thermoplastic resin sheet in this case may preferably be 70% or higher, more preferably 80% or higher, and still more preferably 90% or higher, and/or the total light transmittance of the thermoplastic resin sheet in this case may preferably be 40% or higher, more preferably 50% or higher, and still more preferably 60% or higher. The haze and total light transmittance are the values measured by a measurement method in accordance with JIS K 7105. Further, fine particles contained in the thermoplastic resin sheet may preferably be dispersed substantially uniformly to diffuse light from a light source uniformly and well.

As the material of fine particle, there can be mentioned, for example, synthetic resins such as (meth)acrylic resins, styrene resins, polyurethane resins, polyester resins, silicone resins, fluorine resins, and copolymers thereof; glass; clay compounds such as smectite and kaolinite; and inorganic oxides such as silica and alumina. In these materials, silicone resins and silica are particularly preferred.

The fine particle may be formed from a single material or from two or more kinds of materials, and may be composed of one kind of fine particle formed from the same material, or two or more kinds of fine particles formed from different materials.

As the shape of fine particles, there can be mentioned, for example, spherical, flat, ellipsoidal, polygonal, and platy. Fine particles having these shapes may be used alone, or two or more kinds of fine particles having these shapes may also be used in combination. In the fine particles having these shapes, spherical particles are preferred; however, non-spherical particles such as flat, ellipsoidal, polygonal, and platy particles may be preferred because they have light diffusion property stronger than that of spherical particles and give high brightness at a small addition amount.

The average particle diameter of fine particles may preferably be not smaller than 0.1 μm and not greater than 30 μm, more preferably not smaller than 0.5 μm and not greater than 25 μm, and still more preferably not smaller than 1 μm and not greater than 20 μm. When the average particle diameter is smaller than 0.1 μm, incident light into a thermoplastic resin sheet cannot sufficiently be diffused in some cases. In contrast, when the average particle diameter is greater than 30 μm, the amount of light passing through a thermoplastic resin sheet may be decreased, thereby lowering brightness. The average particle diameter of fine particles is a value obtained by simply averaging the particle diameters of arbitrary hundred fine particles observed with a microscope. Further, when fine particles are non-spherical, an average of the greatest diameter and the smallest diameter is defined as their particle diameter.

The amount of fine particles to be used may preferably be not smaller than 0.1 parts by mass and not greater than 20 parts by mass, more preferably not smaller than 0.2 parts by mass and not greater than 10 parts, relative to 100 parts by mass of a thermoplastic resin constituting a sheet. When the amount of fine particles to be used is smaller than 0.1 parts by mass, incident light into a thermoplastic resin sheet cannot sufficiently be diffused in some cases. In contrast, when the amount of fine particles to be used is greater than 20 parts by mass, the extrusion of a thermoplastic resin sheet may become difficult, and the amount of light passing through a thermoplastic resin sheet is decreased, thereby lowering brightness.

A polycarbonate resin particularly preferred as a thermoplastic resin constituting a sheet can be obtained by, for example, the reaction of a divalent phenol with a carbonate precursor using an interfacial polycondensation method or a melting method.

As the divalent phenol, there can be mentioned, for example, 2,2-bis(4-hydroxyphenyl)propane (commonly known as bisphenol A), 1,1-bis(4-hydroxy-phenyl)ethane, 1,1-bis(4-hydroxyphenyl)cyclohexane, 2,2-bis(3-methyl-4-hydroxyphenyl)propane, 2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)propane, bis-(4-hydroxyphenyl)sulfide, and bis(4-hydroxyphenyl)-sulfone. These divalent phenols may be used alone, or two or more kinds of these divalent phenols may also be used in combination. In these divalent phenols, bisphenol A may particularly be preferred.

Further, as the carbonate precursor, there can be mentioned, for example, carbonyl halides, carbonate esters, and haloformates, and specifically, for example, phosgene, diphenyl carbonate, or dihaloformate of divalent phenol is used.

When a polycarbonate resin is produced by the reaction of a divalent phenol with a carbonate precursor using an interfacial polymerization method or a melting method, a catalyst(s), an end stopping agent(s), and an antioxidant(s) such as divalent phenols may be used, if necessary.

Further, a polycarbonate resin may be a branched polycarbonate resin in which a multifunctional aromatic compound having three or more functionalities is copolymerized, a polyester carbonate resin in which a bifunctional aromatic or aliphatic carboxylic acid is copolymerized, or a mixture of two or more kinds of polycarbonate resins obtained.

The molecular weight of a polycarbonate resin, expressed as a viscosity-average molecular weight, may preferably be not smaller than 15,000 and not greater than 40,000, more preferably not smaller than 18,000 and not greater than 35,000. The viscosity-average molecular weight is a value obtained as follows: specific viscosity (ηsp) determined from a solution containing 0.7 g of polycarbonate resin dissolved in 100 mL of methylene chloride at 20° C. is inserted in the following formula: ηsp/c=[η]+0.45×[η]²c [η]=1.23×10⁻⁴M^(0.83) wherein c is equal to 0.7, [η] is the limiting viscosity, and M is the viscosity-average molecular weight.

To a polycarbonate resin, an additive(s) may be added, if necessary, at an addition amount(s) enough to exhibit its (their) performance(s), for example, heat stabilizers such as phosphorous acid, phosphoric acid, phosphorous acid esters, phosphoric acid esters, and phosphonic acid esters; ultraviolet absorbers such as those of triazole type, acetophenone type, and salicylic acid ester type; antistatic agents such as surfactants of anionic type, cationic type, ampholytic type, and nonionic type, and electrically conductive resins; bluing agents such as anthraquinone dyes; flame retardants such as tetrabromobisphenol A, low molecular weight polycarbonates of tetrabromo-bisphenol A, and decabromodiphenylene ether; and flame retardant aids such as antimony trioxide.

As described above, the production process of the present invention is a process in which a thermoplastic resin sheet is extruded, while controlled warpage is given to the extruded sheet depending on the height, shapes, and temperatures of warpage-controlling rolls.

The conditions of extrusion in the production process of the present invention, for example, a discharge rate from a die, temperature and distance of cooling rolls, and take-up speed of take-up rolls, may be set to those which are substantially the same as in the case where a flat thermoplastic resin sheet is produced, although they are not particularly limited. However, it is necessary to set a temperature of a sheet at a position of warpage-controlling rolls to be around a glass transition temperature (Tg) of a thermoplastic resin constituting the sheet, preferably within a range of from (Tg−20° C.) to (Tg+20° C.), by adjusting a discharge rate from a die and/or by using a heating means, for example, any of various kinds of heaters. In general, regarding a position where a temperature of the extruded sheet becomes around a glass transition temperature (Tg) of a thermoplastic resin constituting the sheet, it shifts downstream in the machine direction of extrusion with an increase in the discharge rate from a die, whereas it shifts upstream in the machine direction of extrusion with a decrease in the discharge rate from a die.

FIG. 1 schematically shows the structure of a typical sheet extruder which can be used in the production process of the present invention. This sheet extruder 10 is an ordinary sheet extruder comprising an extruding apparatus (not shown in the figure), a die 11, a first cooling roll 12, a second cooling roll 13, a third cooling roll 14, guide rolls 15, and take-up rolls 16, and in addition, a pair of warpage-controlling upper and lower rolls 17 is disposed between the third cooling roll 14 and the guide rolls 15. The warpage-controlling rolls 17 can be shifted on an upper side or a lower side to adjust their height.

When a thermoplastic resin sheet is produced using the sheet extruder shown in FIG. 1, for example, a heating means for heating the sheet, a cooling means for cooling the sheet, and if necessary, a heat retention means for heat retaining the sheet, may be disposed between the warpage-controlling rolls 17 and the guide rolls 15, preferably just downstream the warpage-controlling rolls 17 in the machine direction of extrusion to adjust a difference in temperatures between the upper and lower surfaces of the sheet, in addition to the shift of the warpage-controlling rolls 17. To set the upper surface of a sheet to be a high temperature side and the lower surface of the sheet to be a low temperature side, a heating means, and if necessary, a heat retention means, are disposed on an upper side of the sheet, and/or a cooling means is disposed on a lower side of the sheet; or to set the upper surface of a sheet to be a low temperature side and the lower surface of the sheet to be a high temperature side, a cooling means is disposed on an upper side of the sheet, and/or a heating means, and if necessary, a heat-retention means, are disposed on a lower side of the sheet. As the heating means, there can be mentioned, for example, electric heaters, infrared heaters, and hot-air blowers, and as the cooling means, there can be mentioned, for example, blowers and cold-air blowers, and as the heat-retaining means, there can be mentioned, for example, heat retention covers, although all of these are not particularly limited.

The steps of producing a thermoplastic resin sheet of the present invention using the sheet extruder 10 shown in FIG. 1 will be described below. First, a thermoplastic resin constituting the sheet, and if necessary, various additives, are supplied to an extruding apparatus (not shown in the figure), and kneaded sufficiently, which is then extruded in the shape of a melt sheet from the die 11. The extruded sheet is introduced between the first cooling roll 12 and the second cooling roll 13 to proceed on the peripheral surface of the second cooling roll 13, and then introduced between the second roll 13 and the third roll 14 to proceed on the peripheral surface of the third cooling roll 14. The sheet is separated from the third cooling roll 14 at a position of a separating line 18, and then allowed to pass through a pair of warpage-controlling upper and lower rolls 17, after which the sheet is taken up through the guide rolls 15 by the take-up rolls 16.

In this case, the position of the warpage-controlling rolls 17 is a position that the temperature of the extruded sheet becomes around a glass transition temperature (Tg) of a thermoplastic resin constituting the sheet, preferably within a range of from (Tg−20° C.) to (Tg+20° C.), and at this position, mechanically controlled warpage is given to the thermoplastic resin sheet 20 by selecting the shapes of the warpage-controlling rolls 17 and/or by disposing the warpage-controlling rolls 17 on an upper side or a lower side of a height where a flat sheet is obtained.

To give controlled warpage depending on the shapes of the warpage-controlling rolls 17, for example, rolls having shapes such as crowning, reverse crowning, or arch may be used as the warpage-controlling rolls 17, thereby giving upward or downward warpage along the lateral direction. In this case, the amount of warpage can be controlled depending on the amount of crowning or bowing in the warpage-controlling rolls 17.

Further, to give controlled warpage depending on the height of the warpage-controlling rolls 17, for example, when the warpage-controlling rolls 17 are disposed on an upper side as shown in FIG. 2, warpage is given upwardly along the longitudinal direction. In contrast, when the warpage-controlling rolls 17 are disposed on a lower side as shown in FIG. 3, warpage is given downwardly along the longitudinal direction. In this case, the amount of warpage can be controlled depending on the height of the warpage-controlling rolls 17 (i.e., a shift distance from a height where a flat sheet is obtained). Specifically, for example, when the warpage-controlling rolls 17 are disposed within a range of about 100 mm or smaller on an upper side or a lower side, based on a height where a flat sheet is obtained, warpage is given upwardly or downwardly within a range of about 100 mm or smaller, respectively.

Further, thermally controlled warpage can also be given by adjusting the respective temperatures of the warpage-controlling rolls 17 to generate a difference in cooling rates on the upper and lower surfaces of a sheet. In this case, the amount of warpage can be controlled depending on a difference in temperatures between the warpage-controlling upper roll 17 and the warpage-controlling lower roll 17.

The glass transition temperature (Tg) of a thermoplastic resin constituting a sheet may be measured beforehand by DSC (differential scanning calorimeter measuring method). Further, since the temperatures on the upper and lower surfaces of a sheet at a position of warpage-controlling rolls are substantially the same as each other, the temperature of an extruded sheet means a temperature on the upper surface of a sheet, which can be measured using a radiation thermometer.

The thermoplastic resin sheet thus obtained has controlled warpage depending on the height, shapes, and temperatures of a pair of warpage-controlling rolls used in extrusion. According to the production process of the present invention, a thermoplastic resin sheet with controlled warpage can be produced simply and easily with high efficiency without providing an additional step of hot forming other than the step of extrusion. Therefore, the production process of the present invention is advantageous from an industrial point of view.

<<Thermoplastic Resin Sheet>>

The thermoplastic resin sheet of the present invention is a thermoplastic resin sheet which can be produced by the above-described process, and it can be utilized, for example, as a light diffusion sheet used in backlight units of liquid crystal display devices or monitors for medical use, or as a building material of buildings and plants. Since it can stabilize the display performance of liquid crystal display devices for a long period of time, it may particularly preferably be used as a light diffusion sheet of backlight units for large size liquid crystal display devices used in over 15-inch liquid crystal television sets or over 15-inch liquid crystal display panels of desk-top personal computers.

EXAMPLES

The present invention will be described below in more detail by way of Examples, but the present invention is not limited to the following Examples. The present invention can be put into practice after appropriate modifications or variations within a range meeting the gists described above and later, all of which are included in the technical scope of the present invention.

First, measurement methods are described for the glass transition temperature (Tg) of a thermoplastic resin constituting a sheet, the temperature of a sheet, and the amount of warpage.

<Glass Transition Temperature (Tg)>

The glass transition temperature (Tg) of a thermoplastic resin was measured in accordance with JIS K 7121 using a differential scanning calorimeter (trade name: Thermo Plus DSC8230, available from Rigaku Corporation). Specifically, a sample was placed in the differential scanning calorimeter, and maintained at 180° C. for 10 minutes. The sample was then rapidly cooled to 80° C., and after stabilizing, raised to 180° C. at a rate of 20° C./min. The glass transition temperature (Tg) was determined from the resultant DSC curve.

<Temperature of Sheet>

The temperature of an extruded sheet was measured using a radiation thermometer (trade name: IR-TAF, available from CHINO Corporation). The temperatures on the upper and lower surfaces of a sheet at a position of warpage-controlling rolls are substantially the same as each other, and the temperature on the upper surface of the sheet was defined as the temperature of the sheet.

<Amount of Warpage>

The amount of warpage was determined as follows: two holes are made at the trisected positions near the edge of one side in the lateral direction of a thermoplastic resin sheet of 1,000 mm length and 1,000 mm width, through which holes a cord was allowed to pass for hanging the sheet down, and on this condition, threads were strung in the centers of opposite sides in the longitudinal and lateral directions of the sheet, and a distance from the center part of the sheet to the threads was measured with a steel scale. When warpage is upwardly convex, the warpage is denoted as “+”, and when warpage is downwardly convex, the warpage is denoted as “+”.

Reference Example

Using a sheet extruder (of which structure is the same as the sheet extruder shown in FIG. 1) equipped with a T-die of 1,200 mm width, a first, second and third cooling rolls of 300 mm diameter, warpage-controlling rolls of 200 mm diameter, guide rolls, and take-up rolls, a polycarbonate resin sheet was produced as follows:

To the above-described sheet extruder, a T-die lip of 120 mm screw diameter was set, a polycarbonate resin (trade name: 302-6, available from Sumitomo Dow Limited; glass transition temperature (Tg), 153° C.) as a thermoplastic resin was continuously extruded at 280° C. in 1,100 mm width, and extended through the first and second cooling rolls by applying pressure (i.e., the both surface touch system) to be formed into a polycarbonate resin sheet while cooling. For example, as shown in FIG. 1, a pair of warpage-controlling upper and lower rolls 17 (of which respective temperatures were 95° C.) was disposed so that the sheet becomes flat from the separating line 18 to the guide rolls 15, and taken up by the take-up rolls 16, thereby giving a polycarbonate resin flat sheet 20. The temperature of the sheet at a position of the warpage-controlling rolls was (Tg+5° C.). The both edges of the sheet obtained were side-trimmed each by 50 mm in the transverse direction to have a width of 1,000 mm, and then cross-cut to have a length of 1,000 mm, thereby giving a polycarbonate resin sheet of 2 mm thickness. The results obtained by the measurement of warpage of the sheet obtained are shown in Table 1.

Example 1

A polycarbonate resin sheet was obtained in the same manner as described in Reference Example, except that a pair of warpage-controlling upper and lower rolls 17 (of which respective temperatures were 95° C.) was disposed so that the sheet became convex upwardly from the separating line 18 to the guide rolls 15, for example, as shown in FIG. 2, and more specifically, the height of the warpage-controlling rolls was shifted by 2 cm on an upper side, based on the height where the flat sheet was obtained. The results obtained by the measurement of warpage of the sheet obtained are shown in Table 1.

Example 2

A polycarbonate resin sheet was obtained in the same manner as described in Reference Example, except that a pair of warpage-controlling upper and lower rolls 17 (of which respective temperatures were 95° C.) was disposed so that the sheet became convex upwardly from the separating line 18 to the guide rolls 15, for example, as shown in FIG. 2, and a roll having a reverse crowning shape was used as the warpage-controlling upper roll and a roll having a crowning shape was used as the warpage-controlling lower roll, and more specifically, the height of the warpage-controlling upper and lower rolls each having a crowning amount of 2 cm was shifted by 2 cm on an upper side, based on the height where the flat sheet was obtained. The temperature of the sheet at the position of the warpage-controlling rolls was (Tg−10° C.). The results obtained by the measurements of warpage of the sheet obtained are shown in Table 1.

Example 3

A polycarbonate resin sheet was obtained in the same manner as described in Reference Example, except that a pair of warpage-controlling upper and lower rolls 17 (of which respective temperatures were 95° C.) was disposed so that the sheet became convex downwardly from the separating line 18 to the guide rolls 15, for example, as shown in FIG. 3, and more specifically, the height of the warpage-controlling rolls was shifted by 4 cm on a lower side, based on the height where the flat sheet was obtained. The temperature of the sheet at the position of warpage-controlling rolls was (Tg−5° C.). The results obtained by the measurement of warpage of the sheet obtained are shown in Table 1.

Example 4

A polycarbonate resin sheet was obtained in the same manner as described in Reference Example, except that a pair of warpage-controlling upper and lower rolls 17 was disposed so that the sheet became convex downwardly from the separating line 18 to the guide rolls 15, for example, as shown in FIG. 3, and a difference in temperatures between the warpage-controlling upper and lower rolls was adjusted, and more specifically, the height of the warpage-controlling rolls (the temperature of the upper roll was 100° C. and the temperature of the lower roll was 80° C.) was shifted by 4 cm on a lower side, based on the height where the flat sheet was obtained. The temperature of the sheet at the position of the warpage-controlling rolls was (Tg+10° C.). The results obtained by the measurement of warpage of the sheet obtained are shown in Table 1.

Example 5

A polystyrene resin sheet was obtained in the same manner as described in Example 1, except that a polystyrene resin (trade name: HH203, available from PS Japan Corporation; glass transition temperature (Tg), 108° C.) was used as the thermoplastic resin, the temperature of extrusion was set at about 220° C., and the respective temperatures of the warpage-controlling rolls 17 were set at 60° C. The results obtained by the measurement of warpage of the sheet obtained are shown in Table 1.

Example 6

A methacrylic resin sheet was obtained in the same manner as described in Example 1, except that a methacrylic resin (trade name: SUMIPEX EX, available from Sumitomo Chemical Co., Ltd.; glass transition temperature (Tg), 97° C.) was used as the thermoplastic resin, the temperature of extrusion was set at about 250° C., and the respective temperatures of the warpage-controlling rolls 17 were set at 80° C. The results obtained by the measurement of warpage of the sheet obtained are shown in Table 1.

Example 7

A polycarbonate resin sheet was obtained in the same manner as described in Example 1, except that a polycarbonate resin (trade name: Iupilon E2000FN, available from Mitsubishi Engineering-Plastics Corporation; glass transition temperature (Tg), 154° C.) was used as the thermoplastic resin, and 0.5% by mass of fine spherical silica particles (trade name: SEAHOSTAR KE-P150, available from Nippon Shokubai CO., Ltd.; average particle diameter, from 1.33 to 1.83 μm) was mixed as fine particles with 100% by mass of the polycarbonate resin. The results obtained by the measurement of warpage of the sheet obtained are shown in Table 1.

Comparative Examples 1 to 3

Polycarbonate resin sheets were obtained in the same manner as described in Examples 1 to 3, except that the temperature of the sheet at the position of the warpage-controlling rolls was set at (Tg+30° C.). The results obtained by the measurement of warpage of the sheet obtained are shown in Table 1.

Comparative Examples 4 to 6

Polycarbonate resin sheets were obtained in the same manner as described in Examples 1 to 3, except that the temperature of the sheet at the position of the warpage-controlling rolls was set at (Tg−30° C.). The results obtained by the measurement of warpage of the sheet obtained are shown in Table 1. TABLE 1 Temperature of sheet at Shift position of distance of Amount of warpage warpage- warpage- Shapes and temperatures of (mm) controlling controlling warpage-controlling rolls Longitudinal Lateral rolls¹⁾ rolls Upper roll Lower roll direction²⁾ direction³⁾ Reference Tg + 5° C. 0 cm flat, 95° C. flat, 95° C. −3 −2 Example Example 1 Tg + 5° C. 2 cm flat, 95° C. flat, 95° C. +18 +3 on upper side Example 2 Tg − 10° C. 2 cm reverse crowning, 95° C. +21 +19 on upper side crowning, 95° C. Example 3 Tg − 5° C. 4 cm flat, 95° C. Flat, 95° C. −39 −3 on lower side Example 4 Tg + 10° C. 4 cm flat, 100° C. flat, 80° C. −53 −20 on lower side Example 5 Tg + 5° C. 2 cm flat, 60° C. flat, 60° C. +19 +2 on upper side Example 6 Tg + 5° C. 2 cm flat, 80° C. flat, 80° C. +21 +3 on upper side Example 7 Tg + 5° C. 2 cm flat, 95° C. Flat, 95° C. +18 +2 on upper side Comparative Tg + 30° C. 2 cm flat, 95° C. flat, 95° C. +2 −2 Example 1 on upper side Comparative Tg + 30° C. 2 cm reverse crowning, 95° C. +2 +2 Example 2 on upper side crowning, 95° C. Comparative Tg + 30° C. 4 cm flat, 95° C. flat, 95° C. −3 −3 Example 3 on lower side Comparative Tg − 30° C. 2 cm flat, 95° C. flat, 95° C. +2 +1 Example 4 on upper side Comparative Tg − 30° C. 2 cm reverse crowning, 95° C. +2 +2 Example 5 on upper side crowning, 95° C. Comparative Tg − 30° C. 4 cm flat, 95° C. flat, 95° C. −4 −3 Example 6 on lower side ¹⁾Tg is the glass transition temperature (153° C.) of the polycarbonate resin. ²⁾The longitudinal direction is the machine direction of extrusion. ³⁾The lateral direction is the transverse direction of extrusion.

As can be seen from Table 1, the thermoplastic resin sheets of Examples 1 to 7 in which the warpage-controlling rolls were disposed, on an upper side or a lower side of the height where the flat sheet was obtained, at a position where the temperature of the extruded sheet was within a range of not lower than (Tg−20° C.) and not higher than (Tg+20° C.), were apparently given mechanically controlled warpage, as compared with the thermoplastic resin sheet of Reference Example in which the warpage-controlling rolls were disposed at the height where the flat sheet was obtained, at the position where the temperature of the extruded sheet was (Tg+5° C.). More specifically, the thermoplastic resin sheets of Examples 1, 3, 5, 6, and 7 in which the warpage-controlling rolls each having a flat shape were used were given upward or downward warpage along the longitudinal direction, the amount of the warpage being controlled corresponding to the shift distance of the warpage-controlling rolls. Moreover, the thermoplastic resin sheet of Example 2 in which rolls having the selected shapes were used as the warpage-controlling rolls was given warpage on the same level along the longitudinal and lateral directions, the amount of the warpage being controlled depending on the shift distance and amount of crowning of the warpage-controlling rolls. Further, the thermoplastic resin sheet of Example 4 in which the respective temperatures of the warpage-controlling rolls were adjusted to generate a difference in cooling rates between the upper and lower surfaces of the sheet was given larger warpage along the longitudinal and lateral directions.

In contrast, the thermoplastic resin sheets of Comparative Examples 1 to 6 in which the warpage-controlling rolls were disposed, on an upper side or a lower side of the height where the flat sheet was obtained, at the position where the temperature of the extruded sheet was within a range of from (Tg−30° C.) to (Tg+30° C.) were not given mechanically controlled warpage because the temperature of the extruded sheet was too low or too high, in the same manner as the thermoplastic resin sheet of Reference Example in which the warpage-controlling rolls were disposed at the height where the flat sheet was obtained, at the position where the temperature of the extruded sheet became (Tg+5° C.).

From the above results, it is understood that when a thermoplastic resin sheet is extruded, mechanically controlled warpage, preferably controlled warpage depending on the height, shapes, and temperature of warpage-controlling rolls, can be given to the sheet at a position where the temperature of the extruded sheet becomes around a glass transition temperature (Tg) of a thermoplastic resin constituting the sheet, preferably within a range of from (Tg−20° C.) to (Tg+20° C.).

INDUSTRIAL APPLICABILITY

The present invention can provide a thermoplastic resin sheet with controlled warpage simply and easily with high efficiency, and therefore, by using such a thermoplastic resin sheet, for example, as a light diffusion sheet in backlight units of liquid crystal display devices and monitors for medical use or as a building material of buildings and plants, the present invention will make a great contribution to a wide range of fields.

The present invention has been fully described by way of Examples, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention defined below, they should be construed as being included therein. The scope of the present invention, therefore, should be determined by the following claims.

The Japanese Patent Laid-open (Kokai) Publications cited above are incorporated herein by reference. 

1. A process for producing a thermoplastic resin sheet with controlled warpage, the process comprising extruding a thermoplastic resin sheet, while giving mechanically controlled warpage to the sheet at a position where a temperature of the extruded sheet becomes around a grass transition temperature (Tg) of a thermoplastic resin constituting the sheet.
 2. The production process according to claim 1, wherein the sheet is allowed to pass through a pair of warpage-controlling upper and lower rolls at the position, and the mechanically controlled warpage is given to the sheet by selecting shapes of the warpage-controlling rolls and/or by disposing the warpage-controlling rolls on an upper side or a lower side of a height where a flat sheet is obtained.
 3. The production process according to claim 2, wherein the temperature of the sheet at a position of the warpage-controlling rolls is within a range of from (Tg−20° C.) to (Tg+20°).
 4. The production process of claim 2, wherein thermally controlled warpage is also given to the sheet by adjusting respective temperatures of the warpage-controlling rolls to generate a difference in cooling rates between upper and lower surfaces of the sheet.
 5. The production process according to claim 2, wherein thermally controlled warpage is also given to the sheet by adjusting a difference in temperatures between upper and lower surfaces of the sheet at a position or in a vicinity of the warpage-controlling rolls to generate a difference in cooling rates between the upper and lower surfaces of the sheet.
 6. The production process according to claim 1, wherein the thermoplastic resin constituting the sheet comprises fine particles.
 7. The production process according to claim 1, wherein the thermoplastic resin constituting the sheet is selected from the group consisting of polycarbonate resins, (meth)acrylic resins, styrene resins, acryl-styrene resins, and norbornene resins.
 8. A thermoplastic resin sheet with controlled warpage, which is obtainable by the production process according to claim
 1. 